1. Field of the Invention
This invention relates generally to surgical methods and apparatus for addressing ischemic cardiomyopathy, and more specifically to methods and apparatus for restoring the architecture and normal function of a mammalian heart.
2. Discussion of the Prior Art
The function of a heart in an animal is primarily to deliver life-supporting oxygenated blood to tissue throughout the body. This function is accomplished in four stages, each relating to a particular chamber of the heart. Initially deoxygenated blood is received in the right auricle of the heart. This deoxygenated blood is pumped by the right ventricle of the heart to the lungs where the blood is oxygenated. The oxygenated blood is initially received in the left auricle of the heart and ultimately pumped by the left ventricle of the heart throughout the body. It can be seen that the left ventricular chamber of the heart is of particular importance in this process as it is relied upon to pump the oxygenated blood initially through a mitral valve into and ultimately throughout the entire vascular system.
A certain percentage of the blood in the left ventricle is pumped during each stroke of the heart. This pumped percentage, commonly referred to as the ejection fraction, is normally about sixty percent. It can be seen that in a heart having a left ventricular volume such as seventy milliliters, an ejection fraction of sixty percent would deliver approximately 42 milliliters of blood into the aorta. A heart with reduced left ventricular volume might have an ejection fraction of only 40% and provide a stroke volume of only 28 millimeters.
Realizing that the heart is part of the body tissue, and the heart muscle also requires oxygenated blood, it can be appreciated that the normal function of the heart is greatly upset by clotting or closure of the coronary arteries. When the coronary arteries are blocked, an associate portion of the heart muscle becomes oxygen-starved and begins to die. This is clinically referred to as a heart attack. Ischemic cardiomyopathy typically occurs as the rest of the heart dilates in an attempt to maintain the heart's output to the body.
As the ischemic area loses its contraction, the area of dilatation is restricted to the remaining muscle. The three regions of typical infraction include, 1) the anterior wall septum and anterolateral wall which are supplied by the anterior descending coronary artery; 2) the septum and inferior wall supplied by the left anterior artery and the right coronary artery which narrows due to the heart's elliptical shape; and 3) the lateral wall supplied by the circumflex artery which perfuses the lateral wall including the papillary muscle attachments to the ventricular wall.
As the ischemic cardiomyopathy progresses, the various structures of the heart are progressively involved including the septum, the apex and the anterolateral wall of the left ventricle. Within a particular wall, the blood starvation begins at the inside of the wall and progresses to the outside of the wall. It can be seen that addressing ischemic cardiomyopathy shortly after the heart attack can limit the detrimental effects to certain elements of the heart structure, as well as the inner most thicknesses of the walls defining those structures.
As a heart muscle is denied blood nourishment support, its ability to participate, let alone aid, in the cardiac pumping function, is greatly diminished and typically nil. Such muscle is commonly referred to as akinetic, meaning it does not move. In some cases the wall will form elastic scar tissue which tends to balloon in response to the pumping action. This muscle tissue is not only akinetic, in that it does not contribute to the pumping function, but it is in fact dyskinetic, in that it detracts from the pumping function.
The akinetic tissue will, in addition to not contracting, cause cardiac enlargement due to dilatation or loss of its contractile capacity. The dilatation will widen, and thereby change the fiber orientation of the remaining muscle in the left ventricle. This will make the ventricle spherical, and change it from the normal elliptical form which optimizes contraction.
The shape of the ventricle is normally elliptical or conical with an apex that allows a 60 degree fiber orientation of the muscle. This orientation ensures efficient development of intramuscular torsion to facilitate the pumping of blood. Compression of the left ventricular cavity occurs by torsional defamation which thickens the left ventricular wall. This increases progressively from the mid-ventricular wall to the apex. As a result, maintenance of the apical anchor is a central theme of cardiac contraction.
Perhaps the most notable symptom of ischemic cardiomyopathy is the reduction in the ejection fraction which may diminish, for example, from a normal sixty percent to only twenty percent. This results clinically in fatigue, and inability to do stressful activities, that require an increase in output of blood from the heart. The normal response of the heart to a reduction in ejection fraction is to increase the size of the ventricle so that the reduced percentage continues to deliver the same amount of oxygenated blood to the body. By way of example, the volume of the left ventricle may double in size. Furthermore, a dilated heart will tend to change its architecture from the normal conical or apical shape, to a generally spherical shape. The output of blood at rest is kept normal, but the capacity to increase output of blood during stress (i.e., exercise, walking) is reduced. Of course, this change in architecture has a dramatic effect on wall thickness, radius, and stress on the heart wall. In particular, it will be noted that absent the normal conical shape, the twisting motion at the apex, which can account for as much as one half of the pumping action, is lost. As a consequence, the more spherical architecture must rely almost totally on the lateral squeezing action to pump blood. This lateral squeezing action is inefficient and very different from the more efficient twisting action of the heart. The change in architecture of the heart will also typically change the structure and ability of the mitral valve to perform its function in the pumping process. Valvular insufficiency can also occur due to dilatation.
A major determinant of both cardiac oxygen requirement and efficiency is based upon a formula where stress or pressure is multiplied by the radius and divided by twice the thickness of the cardiac wall. Increasing stress reduces contractility or rejecting capacity, and raises energy requirements in the remaining contracting muscle. As the shape changes from elliptical to spherical, wall stress increases thereby demanding higher energy from the remaining cardiac muscle. This dilation, which occurs anteriorly, effects the septum, apex and anterolateral wall. Thus, the normally oval apex becomes more spherical due to 1) a loss of infarcted muscle, and 2) dilation of the remaining contracting muscle.
With inferior coronary artery involvement, the inferior wall, septum, and apex are affected. These elements form, naturally a myocardial triangle, with a base adjacent to the mitral valve, and the septum and free lateral walls forming the planes going to the cardiac apex. As the triangle becomes widened, due to loss of contracting muscle after infraction, the same form of ventricular dilatation occurs. However, instead of making the oval ventricle into a sphere in the anterior segment, with subsequent enlargement (dilatation) of the non-infarcted remaining contracting muscle, there is an increase in the triangle inferiorly. As a result, there is an increase in both the transverse diameter as well as the longitudinal dimension. Thus, inferior coronary involvement results in dilatation of the entire inferior segment.
Although the dilated heart may be capable of sustaining life, it is significantly stressed and rapidly approaches a stage where it can no longer pump blood effectively. In this stage, commonly referred to as congestive heart failure, the heart becomes distended and is generally incapable of pumping blood returning from the lungs. This further results in lung congestion and fatigue. Congestive heart failure is a major cause of death and disability in the United States where approximately 400,000 cases occur annually.
Following coronary occlusion, successful acute reperfusion by thrombolysis, (clot dissolution) percutaneous angioplasty, or urgent surgery can decrease early mortality by reducing arrhythmias and cardiogenic shock. It is also known that addressing ischemic cardiomyopathy in the acute phase, for example with reperfusion, may salvage the epicardial surface. Although the myocardium may be rendered akinetic, at least it is not dyskinetic. Post-infarction surgical revascularization can be directed at remote viable muscle to reduce ischemia. However, it does not address the anatomical consequences of the akinetic region of the heart that is scarred. Despite these techniques for monitoring ischemia, cardiac dilation and subsequent heart failure continue to occur in approximately fifty percent of post-infarction patients discharged from the hospital.
The distribution of heart failure is more common with occlusion of the left anterior descending coronary artery (LAD) due to its perfusion of the apex. But, this can also occur with inferior infarction, especially if there is inadequate blood supply to the apex due to 1) prior damage to the left anterior descending artery, or 2) inadequate blood supply due to stenosis or poor function. In general, the distribution of ischemia is 45% anterior, 40% inferior, and 15% circumflex. However, the incidence of congestive heart failure is more common in the anterior infarction.
Various surgical approaches have been taken primarily to reduce the ventricular volume. This is also intended to increase the ejection fraction of the heart. In accordance with one procedure, viable muscle is removed from the heart in an attempt to merely reduce its volume. This procedure, which is typically accomplished on a beating heart, has been used for hearts that have not experienced coronary disease, but nevertheless, have dilated due to leaking heart valves. Other attempts have been made to remove the scarred portion of the heart and to close the resulting incision. This has also had the effect of reducing the ventricular volume.
In a further procedure, a round, circular patch has been proposed for placement typically in the lateral ventricular wall. Unfortunately, providing the patch with a circular shape has allowed the dilated heart to remain somewhat enlarged with a thin and over-stressed wall section. The exact placement of the patch has been visually determined using only a visual indication where the typically white scar tissue meets the typically red normal tissue. Location of the patch has been facilitated in a further procedure where a continuous suture has been placed around the ventricular wall to define a neck for receiving the patch. The neck has been formed in the white scar tissue rather than the soft viable muscle. This procedure has relied on cardioplegia methods to stop the beating of the heart and to aid in suture placement.
In the past, the patch has been provided with a fixed or semi-rigid wall which has prevented the muscle from becoming reduced to an apical anchor which facilitates the twisting motion. The patches have had a fixed planar configuration which have prevented the lateral muscle from coapting to form an apex.
These surgical procedures have been met with some success as the ejection fraction has been increased, for example, from twenty-four percent to forty-two percent. However, despite this level of success, little attention has been paid to myocardial protection, the potential for monitoring the writhing action associated with apical structure, or the preferred structure for the patch. Failure to protect the heart during restoration of the segment has increased hospital mortality, morbidity, and irreversibly damaged some normal muscle needed to maintain the heart's output.